1. Field of the Invention
This invention relates to an architectural electrochromic pane; more specifically, the invention relates to a building pane, of which the transmission, reflection and energy-absorption factors can be modified by an electrical control.
2. Discussion of the Background
For several years now, the development of air-conditioning systems has created an increasing demand for panes having a low solar gain factor, or in other words glazing panes which contribute as little as possible to the rise in temperature inside the building. For this purpose, architects have turned towards panes comprising a reflective glass for solar protection produced by deposition of a mineral film onto a clear glass or glass tinted within the mass, the deposition being carried out, for example, by a pyrolysis technique or a cathodic sputtering technique under vacuum of a film of metallic oxides.
Unfortunately, the immediate effect of these solar protection films is a reduction in the light transmission which, in the case of the most efficient products, from the point of view of the solar gain factor, requires the almost permanent need for artificial lighting inside the building. It will certainly be understood that this decrease in light transmission is to a large degree unavoidable if it is compared with the physical definition of the solar factor of a pane. That is to say, the solar factor is the ratio of the total energy entering the room through the pane, to the incident solar energy, the total energy being the sum of the solar energy entering by direct transmission and of the energy given up by the pane to the internal environment as a consequence of its heating up by energy absorption. In practice, furthermore, the user makes his choice above all on the basis of the light transmission level, the solar factor being a fallout from this choice.
On the other hand, the sunshine conditions vary from one period of the year to the other and according to the time of day. At the approach to nightfall or in winter in cold or temperate climates, the first concern is a high transmission of light. In contrast, during periods of high sunshine, a sufficient illumination can be obtained even with a very low light transmission percentage; the important thing then is certainly the solar reduction factor, in order to reduce the cost of air conditioning. Finally, and independently of all the sunshine conditions, it may be useful to darken a glazing pane, for example during a video projection.
It was, notably, with the objective of providing an evolutionary solution that electrochromic panes have been developed, that is to say very generally, panes having electrically controlled optical properties. To date, various electrochromic systems have been proposed for applications such as display devices, rear view mirrors with an antidazzle position or again automobile roofs, but no building glazing pane has been fully developed, since this application has always appeared in the technical literature as a still remote objective, not likely to be realized in the short term.
According to an article that appeared in "Solar Energy Materials", volume 22, no. 1, March 1991, Susan Reilly, Deriush Arnateh and Stephen Selkowitz have developed, from calculated models, what they believe to be the optimum configurations for the purpose of reducing the air-conditioning (cooling) and illumination loads. Although this article refers to purely theoretical electrochromic systems, they concluded that in the case of a monolithic pane it is preferable to place the electrochromic system on the face which is towards the outside, whereas in the case of double glazing, the system should preferably be placed on the panel which is towards the outside, but facing towards the intermediate layer of air. Furthermore, these authors mention the advantage of incorporating, in addition, one or more "static" selective films, such as low-emissivity films or sheets of tinted glass.
This article does not, however, take into account the practical constraints of the production of a building pane nor of the fact that an electrochromic system should, if possible, be placed in specific conditions suitable for preventing its degradation.